(If you missed the last post highlighting the major design choices for the tiny hacker house, this one may be a little out of context. You should probably go read it now.)

When I first contemplated building a tiny house, the electricity system was the first thing I researched. Maybe that’s because it’s what I’m most comfortable with as an electrical / computer engineer, or perhaps it’s because that’s what enables the rest of the house to come alive. Since I wasn’t planning to be grid-tied, I had to depend on my electrical system to provide the comforts for everyday life. If I wanted to rely on my home to support a decent living, it needed to be fault-tolerant and straightforward to manage.

After much research, I’ve chosen to go with a solar system and propane-converted Honda generator for backup. The panels will lie flat on my slanted roof and I’ll park the house facing south whenever possible to maximum sunlight absorption. I toyed with the idea of building a collapsible windmill like this one for backup power, but it would be need to be huge, durable, and powerful – likely a too expensive and time-consuming job for a tiny house. I’ll just stick to solar and propane. At least until my nuclear reactor arrives in the mail.

Power Solar Panels
You could say I'm a bit power hungry.

Tiny houses are already space-confined; I didn’t want to feel electrically-confined as well. At the very minimum, I knew I needed to power A/C, a refrigerator, lights, internet equipment, and a computer simultaneously. At the peak, however, that includes a few more things – pressurizing water, brewing coffee, and using a hair dryer perhaps. I wanted to be able to support a realistic load without tripping the breaker. So I came up with the following table:

Device Max power draw (W)
Laptop 85
Hairdryer 1000
Coffee machine 800
TV 100
Air conditioner 800
Refrigerator 65
Wireless router 10
Lights (4) 60
Water pressurizer pump 90
Total 3010

Knowing that I’d need up to 3010 W of power at any one time, I could size my inverter. If you’re not familiar with electrical systems, the inverter is the device that takes power from a 12 V DC source like a car battery and converts it to the 120 V or 240 V AC standards used in homes around the world. They’re never 100% efficient, so a little power is lost in the form of heat during the conversion process. They also don’t like being run at maximum capacity for extended periods of time, so it’s good to leave a buffer to account for unexpected power draw. Because of this (and other reasons noted below), I decided to go with Outback’s FlexPower One utilizing a 3600 W inverter.

To continue sizing the rest of the solar system, I needed to calculate my average kWh used per day. A kWh, or kilowatt-hour, of electricity is simply the amount of kilowatts used in one hour. To calculate this I determine the number of hours I expect to use each device in a given day, multiply by its power draw from the table above, and sum the result:

Device Usage per Day (h) Power usage (Wh)
Laptop 4 340
TV 2 200
Air conditioner 4 3200
Refrigerator 8 520
Wireless router 24 240
Lights (4) 6 360
Water pressurizer pump 2 180
  Total 5040 Watt-hours

5040 Wh or about 5 kWh of power used in a typical day. To account for heat loss inefficiencies in the system I multiplied this number by 1.5, ending up with 7.5 kWh. This means the battery bank will need to supply 7.5 kWh per day if fully drained, but since it’s damaging to discharge them more than 50%, we need to multiply this number by 2. So 15 kWh of electricity is needed.

To find the number of deep-cycle batteries I’ll need, I divide 15 kWh by the voltage of my battery bank, 48 V. This gives me the required Ampere-hour (Ah) rating of the bank. These heavy duty deep cycle batteries offer 1240 Ah at 12 V, or 310 Ah at 48 V. This comes pretty close to the 312.5 Ah required, and considering I’ll have a 3000 W generator backup, should be adequate for the tiny hacker house. For a better how-to on sizing a solar battery bank, check this Instructables DIY.

Honda Generator
A 3000 W propane-fueled generator sits on a hitch-mounted luggage rack on the back of the trailer.

Finally, I need to be fairly confident the solar panel array can provide enough juice to charge the battery banks to 100% on a clear, sunny day. This will vary wildly depending on the time of year, where I’m parked, the weather, angle of the panel, and efficiency of the panel. For example, using this panel sizing calculator, Phoenix, AZ receives about 5.75 sun-hours in the winter, while Seattle, WA only musters 1.6. This means your solar panel array would need to be about 3.5 times larger if living in Seattle than Phoenix.

For the tiny hacker house, the majority of power is used by the A/C, so in the winter months my power needs will be reduced to about 1.8 kWh – less than half required in the summer months. Heating will be provided by in-floor solar heating with a propane stove as backup, so minimal electricity requirements there. I should mention that insulation plays a huge role in the power requirements to heat and cool the home. Closed-cell spray foam insulation seems to be the best available, which I’ll cover in a later article.

Solar panels
At 21.3" x 47", these Renogy 100 W panels fit perfectly on the roof.

The tiny hacker house roof has room for about 800 W of panels. Taking everything into consideration, in the worst case scenario, I’d be able to produce 0.8 kW * 1.6 h = 1.28 kWh / day on average during the winter in Seattle. At an estimated usage of 1.8 kWh / day with no A/C, that means my 3000 W generator will need to run at full load for (1.8 - 1.28) / 3 = 0.1733 hours, or about 10.4 minutes per day to make up the difference the solar panels can’t provide. I’m ok with that.

As you can tell, designing and managing an off-grid electrical system is pretty complicated. That’s why I chose to go with an integrated power management system like the FlexPower One. It provides an inverter, MPPT charge controller, logging device, and will even remote-start my generator when the battery bank falls below a supplied charge threshold. With 100 lb of propane, I could run the generator at full load for up to 40 hours (calculated from here). Plenty of power to live comfortably without worry!

That sums up the electrical system – hope you made it all the way through. If you have any questions please leave a comment below or shoot me an email. In the next post we’ll dip into some smelly subjects as we take a splash in the bathroom. :-)

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